1 Field of the Invention
This invention relates to powder metallurgy, and in particular to the application of powder metallurgy to produce precisely repositionable components.
2. Discussion of the Prior Art
International Patent Publication No. WO 97/42424 published Nov. 13, 1997, which is hereby incorporated by reference, discloses an integral dowel design solution for a problem where there was a specific need for bearing caps to be accurately repositioned after joint separation and reassembly. U.S. patent application Ser. No. 09/155,781 filed Oct. 2, 1998, which issued Jul. 11, 2000 as U.S. Pat. No. 6,086,258, hereby incorporated by reference, is the national phase in the United States of International Patent Application Serial No. PCT/US97/04050 filed Mar. 12, 1997 which was published in the above identified International Publication No. WO 97/42424.
The essential function of a bearing cap is to retain and locate a rotary shaft, or a bearing for a rotary shaft which in turn retains and locates the shaft, relative to a support structure. For example, the main bearing cap of an engine bolts to a bulkhead of the engine crankcase and together with the bulkhead retains and locates the crankshaft journal in place while the crankshaft is rotating. The crankshaft journal runs against two half shell bearings which are fitted to the main bearing cap and the engine bulkhead semi-circular bores, respectively.
In this case, for vibration free, low friction and quiet running, the roundness of the bore produced by the main bearing cap and the bulkhead is very important. This roundness is achieved by a machining operation called line boring. The main bearing caps are bolted to the bulkheads of the engine block, and then a boring bar fitted with a cutting tool is used to machine the bores in the assembly. This ensures the two half rounds formed by the main bearing cap and the bearing block form as near to a perfect circle as possible. A finishing operation involving a grinding hone is often used to achieve the extremely fine tolerances needed for quiet running and efficient engine performance.
However, to install the crankshaft, it is necessary to remove the main bearing caps from the engine block. After the crankshaft is put in place, it is necessary to reposition the main bearing caps to the bulkhead so that they are replaced in the identical position they occupied during the line boring operation. Any deviation from that original position produces an out-of-round condition that, in turn, leads to vibration, noise and possibly stiff, high friction crankshaft operation.
There are a number of conventional structures for re-locating and attaching the main bearing caps to bulkheads when installing the crankshaft. One such structure is shown in FIG. 1. In this instance, the main bearing cap C has a very precisely machined, snap-width W, which is the distance across the long axis of the main bearing cap across the foot sections T of the bearing cap. Similarly, a precision channel P is machined in the engine block bulkhead B to produce a controlled interference fit with the feet T when the main bearing cap C is refitted after crankshaft installation.
This method does not, however, provide relocation in the fore and aft direction (i.e., in the direction of the axis of the journal bore J). The bolt holes H themselves are used to control the axial repositioning, and since there is a substantial clearance between the bolts F and the bolt holes H of the main bearing cap C, this relocation accuracy is generally poor.
In addition, the interference fit between the main bearings caps C and the channel P in the engine block B in this structure is a variable which affects the final roundness of the bore J after re-installation. A highly stressed main bearing cap C may stress relieve during engine operation, thereby changing the roundness of the bore. Also, the precision machining operations required on the main bearing caps C to define the snap width W and on the block B to form the channel P, so as to avoid an overstressed or loose main bearing cap in this structure, are relatively expensive.
Another known method of location and attachment is shown in FIG. 2. This involves the use of hollow dowels D. These dowels D are pressed into counter-based holes L in the engine block bulkhead B. The dowels D then locate in precisely machined counterbores M in the corresponding main bearing cap foot sections T. The accuracy of installation of the hollow dowels D is dependent upon the precision counterboring of the engine block and the main bearing cap. Both of these operations have a finite tolerance which, when stacked up with the tolerance on the dowel D outer diameter, can produce an unacceptable variation in location of the main bearing cap C. Additionally, this procedure has the added expense of purchasing precision hollow dowels, their handling and installation, and the costly machining of precision bores L in the bulkhead B and M in the main bearing caps C.
In many cases where hollow dowels as shown in FIG. 2 are used, the engine block channel/main bearing cap snap width relocation method of FIG. 1 is also used. This combination is expensive and, in fact, can produce a situation where the interference fits between the snap-width and channel are in conflict with the interference fits between the hollow dowels and the main bearing cap or bulkhead holes.
It has also become clear that there are many other applications that would benefit from an integral dowel design. One example concerns the need for precise angular location of a toothed sensor ring that measures the timing of an internal combustion engine. FIGS. 27 and 28 show drawings of a portion of the sensor ring and the flywheel or other component to which it is assembled. The previous design of FIGS. 27 and 28 used bolts 601 with a conical head shape that locates into a similar cone shape in a ring 602. This suffers from the problem of using the threaded hole 603 to provide angular location. As stated above, it is well known in the engineering profession that using a threaded hole to both fix and precisely locate two components is not good practice. The reasons are that it is difficult to thread a hole concentrically, and even harder to ensure that the bolt is concentric to the threads.
This stack-up of errors reduces the precision of the fixture to the point where a separate locating dowel 604 is often needed, as illustrated in FIGS. 28a-d, similar to the separate dowel of FIG. 2. As stated above, the two components must be precisely oriented and clamped, then a precision hole 605 must be bored through one component into the second one. Finally, a separate dowel 604 must be pushed through both holes 605 to achieve the desired location precision. This is expensive both in cost of machining and the purchase of the dowel 604.
The present invention provides a structure and method of permitting precise repositioning of two components relative to one another where one of the components is made by powder metallurgy (P/M). The P/M component has an integral boss protruding from it, which is received in a bore of the part to which the component is assembled. The boss is of a shape and ductility so that at least one of the boss and bore plastically conform to one another when they are brought together with force, for example in a pressing operation or when they are bolted together for the first time. The plastic deformation of the boss and bore creates a unique mating surface fit between the two parts so that when the two parts are taken apart and then put back together, they go back together in the exact same, or near to the exact same, position.
In a preferred form, the boss is provided around a bolt hole in the P/M component, and the boss fits into a counterbore of a bolt hole in the part to which the P/M component is assembled. Counterboring bolt holes is a standard process in manufacturing and so the invention is readily adapted to be used without major production line changes.
The boss is preferably tapered, so as to progressively tighten in the bore as it is forced in. A lead-in radius maybe provided on a leading edge of the boss to help initially locate the boss in the bore. Axial splines may be provided on the outside of the boss to further contribute to unique plastic deformation between the boss and bore, with the splines and boss conforming to the bore if the bore is in a relatively hard material such as cast iron, or bite into the bore if the bore is in a relatively soft material such as an aluminum alloy.
Plastic conformance between the bore and the boss is facilitated by the boss and remainder of the bearing cap being sintered powder metal, which is not fully dense. However, it may also need to be ductile, depending on the material of the bore, and if so it is preferably a liquid phase sintering powder metal material. Such a material preferably is a powder metal alloy of iron containing phosphorus from ferrophosphorus powder with a phosphorus content of 0.4 to 0.7% and a carbon content of 0 to 0.8%. Additional strength may be achieved with the addition of copper in the amount of 0 to 4% without loss of ductility.
In another preferred aspect, a moat is formed around a trailing end of the boss. The moat creates a void into which material around the bore may bulge or expand when it is deformed by the insertion of the boss.
In another aspect, the boss may be oblong in one direction, so as to provide an interference fit with the bore in that direction. Other means may be provided to accurately position the components in the other direction.
These aspects may be applied to any of a number of different components. Component applications specifically described are main bearing cap, timing sensor ring and connecting rod bearing cap applications, but the invention is not limited to only these applications.
In another aspect of the invention, a deformable boss can be formed on a powder metal insert for casting which acts as a crush ring to seal molten casting metal from flowing into a hole or crevice which the boss surrounds. The insert is placed in the casting mold, and when the mold halves come together, the bosses are crushed so as to form the seal.
In a method of the invention, two parts, one of which is sintered powder metal, are brought together with enough force to cause plastic conformance between the boss of the P/M part and the hole into which it is inserted. The parts are taken apart and, when reassembled, go back together to replicate the original assembled position.
Other objects and advantages of the invention will be apparent from the detailed description and drawings.